1. Field of the Invention
The invention relates to a method for operating a premix burner and to a corresponding premix burner for carrying out the method.
2. Discussion of Background
Combustion chambers with premix burners which are designed as so-called double-cone burners and in which the fuel is supplied from the outside by plug-in fuel lances have long proven suitable for stationary gas turbines in power plants. The lance is generally configured as a two-fuel lance, i.e. it is possible, as desired, to supply gaseous fuel, e.g. pilot gas, and/or liquid fuel, for example an oil/water mixture. To this end, a liquid-fuel pipe, an atomizer pipe and a pilot-gas pipe are arranged concentrically in the lance. The pipes each form a duct for the liquid fuel, the atomizer air and the pilot gas, which ducts, at the lance head, end in a central fuel nozzle. The head of the fuel lance projects into a corresponding inner pipe of the double-cone burner, so that the fuel emerging passes centrally into the burner inner chamber which adjoins the inner pipe (cf. DE 43 06 956 A1).
EP 0,321,809 B1 has also disclosed a double-cone burner which is provided for use in a combustion chamber which is connected to a gas turbine. This burner comprises two hollow part bodies which complement one another to form the double-cone burner and are arranged radially offset with respect to one another. It has a hollow-cone-shaped inner chamber which increases in size in the direction of flow and has tangential air-inlet slots. The fuel is supplied to the double-cone burner from the outside via the fuel lance which opens out into the central liquid-fuel nozzle. The latter forms a hollow-cone-shaped fuel spray, consisting of liquid fuel and air, in the burner inner chamber, in which spray most of the fuel droplets are concentrated at the outer end of the conical spray pattern.
Owing to the large injection angle of approx. 30.degree. and the absence of an axial impulse in the center, these sprays are highly susceptible to centrifugal forces which are generated by the turbulent flow in the interior of the burner. As a result, the fuel droplets are carried relatively quickly outward by centrifugal forces, resulting, under certain operating conditions, in a not insignificant quantity of the liquid fuel striking the inner walls of the burner.
To atomize liquid fuels, inter alia so-called plain-jet atomizers are also used, which atomizers produce a conical plain jet of uniformly distributed fuel droplets. Such a solution is known from the textbook "Atomization and sprays", by A. Lefebvre, West Lafayette, Indiana 1989, pp. 106/107, 238-241. In the case of this atomizer nozzle, the liquid fuel is ejected at high pressure from an antechamber through a small, circular injection opening of a defined guide length. As a result, the plain-jet atomizer produces a fuel jet with an injection angle of approximately 5.degree. to 15.degree..
However, owing to this small injection angle and the fact that the associated atomization only takes place further downstream, such plain-jet atomizers are not used in combustion chambers of gas turbine installations which are fitted with premix burners, since they require rapid atomization of the liquid fuel. In addition, the plain-jet atomizer described is not particularly suitable for numerous combustion applications, since it has a tendency to concentrate the fuel drops in a small area directly downstream of the nozzle. Particularly under the unfavorable conditions of a low air/fuel ratio and at a low air speed, it is not possible to achieve a sufficient level of atomization.